Top-ranked Serena Williams won in her first match since defaulting in a doubles match with sister Venus at Wimbledon, beating Karolina Pliskova 7-5, 6-2 on Wednesday in the second round of the Bank of the West Classic.
At Wimbledon, Williams tried to play through a viral infection that caused her to lose her equilibrium during the doubles match. On Wednesday night, encouraged by a large crowd, Williams fought through a tough first set before she found her legs and took control of the match with a big second set.
"I thought I played well,'' Williams said. "I'm on the right path, I'm excited and I'm looking forward to my next match.''
In the quarter-finals, Williams will meet the winner of Thursday's match between fifth-seeded Ana Ivanovic and US qualifier Carol Zhao.
Williams casually deflected questions about her health, preferring to bring the conversation into the present.
Earlier, Varvara Lepchenko beat second-seeded Angieszka Radwanska 6-3, 3-6, 6-4 to advance to the quarter-finals.
Lepchenko beat the fifth-ranked Radwanska for the first time in six meetings, edging a top-10 player for only the third time in 24 matches.
Radwanska, the runner-up last year, became the highest seeded player to fall in the tournament.
Lepchenko will face Sachia Vickery, the American qualifier who beat Monica Puig 6-7 (4), 6-2, 6-1 to reach her first career WTA Tour quarter-final.
Petkovic will next meet the winner of Thursday night's match between Venus Williams and Victoria Azarenka.